The present invention is directed to the aircraft flight control art and, in particular, to an active control system for suppressing aircraft body vibrations due to natural mode excitations on multiple geometric axis.
Generally speaking, the total motion of an airplane is understood by those in the art, as comprising; 1) rigid body motion and 2) flexible motion. The rigid body of airplane includes translation and rotation about each of the airplane axis. The flexible motion includes vertical bending of fuselage or wings, lateral bending of fuselage or empenage, and torsional motion of fuselage or wings. As airplanes become lighter, larger and longer, the effects of airplane flexibility on passenger comfort and handling quality becomes more critical and attracts higher attention. Large aircraft, particularly those whose fuselage bodies have a high length to diameter ratio, are subject to gust-induced excitation of natural modes of the aircraft body. These modes include natural bending and twisting of the body in the fore and aft sections of the aircraft. These natural mode excitations are objectionable in commercial aircraft since they can result in passenger and crew ride discomfort or increase in structural load. However, in some severe cases, the pilots might misjudge the vibrations as a limit cycle caused by control oscillatory failure and perform unnecessary corrective actions. Further, when the flight deck pilot station has a lot of vibration, the pilot may unintentionally couple with the aircraft's motion, thereby involuntarily putting in control inputs to sustain or amplify the vibrations to higher levels. This phenomenon is known in the art as “Pilot Induced Oscillation” or “Aircraft-Pilot Coupling” or “Aircraft-Pilot Adverse Coupling”.
Most modem commercial aircraft flight control systems have either a passive or active modal suppression systems. Passive systems have evolved from the earliest designs and are still used, in the interest of economy and simplicity, and usually when the aircraft's vibration level is low such that a more sophisticated approach is not warranted. These systems were typified by their use of control path notch filters to avoid control signal commands at the aircraft structures—structural mode frequency (natural resonance frequency). However, aircrafts having a more complex structural vibration problem typically require the more sophisticated active systems. These systems sense and process feedback signals from appropriate aircraft body locations and move the control surfaces to generate forces that reduce such vibration levels. Most passive and active modal suppression systems, up until now, reduce structural vibrations in a single geometric axis for its fundamental harmonic mode, first body bending mode for example.
Many prior passive and active systems have addressed a variety of aircraft structural modal suppression problems:
(U.S. Pat. No. 2,718,365 White “Deformation Compensator for Aircraft Controls”) A means to increase the effectiveness of the horizontal tail on the longitudinal axis in the presence of fuselage bending;
(U.S. Pat. No. 2,832,551 Gille “Automatic Pilot for Aircraft”) A system for damping lateral fuselage accelerations using rudder deflections;
(U.S. Pat. No. 2,985,409 Atwood et al. “Gust Alleviation System”) A system for “canceling or diminishing the unpleasant effects on passengers” of gust induced vertical accelerations;
(U.S. Pat. No. 3,079,108 Falkner et al. “Control Apparatus”)A system for suppressing aircraft fuselage body bending modes;
(U.S. Pat. No. 3,240,447 Olshausen “Vehicle Control System”) A longitudinal gust alleviation system that uses canards to produce compensatory changes in lift without affecting the crafts pitching moment
(U.S. Pat. No. 3,241,077 Smyth et al. “Self-Adaptive Control System Eliminating Variable Unwanted Component”) A variable notch filter used for decoupling resonant loads from the control system over broad dynamic range;
(U.S. Pat. No. 3,279,725 Andrew et al. “Flight Controller for Flexible Vehicles”) Two actuators, mutually arranged for providing pitching moments of opposite sense, to reduce longitudinal body bending in flexible aircraft;
(U.S. Pat. No. 3,347,498 Priestly et al. “Aircraft Structural Stress Alleviators”) Wing control surfaces to reduce wing structure fatigue caused by wing root body bending;
(U.S. Pat. No. 3,636,324 Dommasch “Air Data Lateral-Directional Stability Augmentation System”) A lateral—directional control system for suppressing lateral axis body bending, that does not use prior inertial sensors.
(U.S. Pat. No. 3,734,432 Low “Suppression of Flutter”) An active control system for suppression of flutter that includes controlling the torsional bending of airfoils. Torsion & bending motions and deflections are sensed and converted by a stability control augmentation system into leading and trailing edge control surface deflections in efforts to counteract said motions & deflections.
(U.S. Pat. No. 3,902,686 Wykes et al. “Combined Vertical and Lateral Identical Location of Accelerometer and Force System”) Lateral accelerometers and control vanes are used to reduce the lateral and vertical vibration at the pilots station.
(U.S. Pat. No. 4,455,004 Whitaker “Flight Control Device for Airplanes”) Wingtip mounted control surfaces provide wing load alleviation during maneuvering & high speed gusts.
(U.S. Pat. No. 4,706,902 Destuynder et al. “Active Method and Installation for the Reduction of Buffeting of the Wings of an Aircraft”) Wing buffeting alleviation system that applies to wing mode flutter control.
(U.S. Pat. No. 4,796,192 Lewis “Maneuver Load Alleviation system”) Wing load alleviation system that selectively deflects aerodynamic surfaces in response to a real-time structural wing load calculation for the purposes of moving the wing lift vector inboard thereby reducing the moment arm by which it is acting.
(U.S. Pat. No. 4,821,981 Gangsaas et al. “Maneuver Enhancement and Gust Alleviation System) A maneuver enhancement & gust load alleviation system that coordinates the control & effects of multiple aircraft control surfaces that are otherwise adversely coupled.
(U.S. Pat. No. 5,186,416 Fabre et al. “System for Reducing the Forces Applied to the Wings and Particularly to the Root of the Wings of an Aircraft in Flight”) Wing load alleviation system that deflects aerodynamic surfaces in response to sensed vertical accelerations for the purposes of reducing wing root bending moments.
(U.S. Pat. No. 5,072,893 Chakravarty et al. “Aircraft Modal Suppression System”) An accelerator & rudder based suppression system to damp aft body lateral accelerations after they occur for the purposes of improving ride quality.
(U.S. Pat. No. 5,224,667 Lacabanne “System Enabling the Flutter Behavior of an Aircraft to be Improved”) An accelerometer and rudder based control system for suppressing lateral bending modes.
(U.S. Pat. No. 5,375,794 Bleeg “Apparatus and Method for Reducing Aircraft Loads Resulting from Atmospheric Turbulence and Gusts”) Pressure sensor & rudder based gust load alleviation system that suppresses weather-vane (beta) rotational modes for the purposes of improving ride quality.
(U.S. Pat. No. 5,452,865 Tran et al. “Aircraft Frequency Adaptive Modal Suppression System”) Modal suppression system that uses dynamic fuel parameter to modify modal suppression control law.
(U.S. Pat. No. 5,598,991 Nadkarni et al. “Method and Apparatus for Detecting Oscillatory Phenomena Indicative of Airflow Separation”) Flow separation monitor improvement to differential pressure sensor based U.S. Pat. No. 5,375,794.
(U.S. Pat. No. 5,667,166 Tran et al. “Aircraft Frequency Adaptive Modal Suppression System”) Modal suppression system that uses gross weight to modify modal suppression control law.
(U.S. Pat. No. 5,669,582 Bryant et al. “Method and Apparatus for Reducing Unwanted Sideways Motion in the Aft Cabin and Roll-Yaw Upsets of an Airplane due to Atmospheric Turbulence and Wind Gusts”) Improvements to U.S. Pat. No. 5,375,794.
(U.S. Pat. No. 5,833,173 Tran et al. “Aircraft Frequency Adaptive Modal Suppression System”) Improvement to U.S. Pat. No. 5,072,893.
From the foregoing discussion, it is well settled in the art to use filters to suppress structural resonance exciting command inputs. Further, it is settled in the art to sense bending & torsional moments of structures caused by gusts, high maneuvering load factors and structural resonances, and to alleviate these upsets through the use of a stability augmentation computer, usually with sensors and flight control effectors in the vicinity of the upset. Moreover, the prior art has been concerned with applications of axis-symmetric flight control inputs, on a particular control axis to counteract a structural upset on the same axis. However, what is not known, concerns the use of anti-symmetric control inputs on one control axis to counteract the effects of control inputs or structural resonances on another control axis.
For these reasons, there is a need for a comprehensive strategy to address structural modal suppression on multiple geometric axis's. Further, there is a need for a simultaneous multi-axis flight control system apparatus wherein all axis flight controls are utilized to alleviate structural resonances of airfoil, aircraft body bending and torsional moments. Still further, there is a need for an active modal suppression control system that is capable of accurately monitoring aircraft structural vibrations due to excitation of natural aircraft bending and torsional modes, wherein the airplane's control surfaces are moved in such a way as to reduce the flexible motion at the same time controlling the rigid body motion.